Simultaneous thermometry and velocimetry for a shock tunnel using homodyne and heterodyne detection

Abstract

At our institute a piston-driven shock tunnel is operated to investigate structures of space transportation systems under reentry and propelled flight conditions. For temperature measurements in the nozzle reservoir under single-shot conditions, laser-induced thermal grating spectroscopy is used to date to measure the speed of sound of the test gas. The temperature then can be calculated from this data. The existing experimental setup has already been successfully used to measure flows up to an enthalpy of 2.1 MJ/kg. Since conducting the experiments is extremely time-consuming, it is desirable to extract as much data as possible from the test runs. To additionally measure the velocity of the test gas, the test setup was extended. Besides, extensive improvements have been implemented to increase the signal-to-noise ratio. As the experiments can be conducted much faster at the double-diaphragm shock tube of the institute without any restrictions on the informative value, the development of the heterodyne detection technique is carried out at this test facility. A series of 36 single-shot temperature and velocity measurements is presented for enthalpies of up to 1.0 MJ/kg. The averaged deviation between the measured values and the values calculated from the shock equations of all measurements related to the average of the calculated values is 2.0% for the Mach number, 0.9% for the velocity after the incident shock and 4.8% for the temperature after the incident shock.

Directly-excited laser-induced thermal grating spectroscopy and thermometry with carbon dioxide vibrational transition

“…Both the electrostrictive grating and thermal grating can generate LIGS signal, and they are sometimes classified as laser-induced electrostrictive grating spectroscopy (LIEGS) and laser-induced thermal grating spectroscopy (LITGS), respectively. Although higher laser energy than LITGS is usually required, the advantage of LIEGS is it does not require molecules that resonate with the laser to generate signals [17] and achieve measurements of temperature [18,19], speed of sound [18] and velocity [19,20]. In the work, the resonant LITGS is the main topic and will be discussed in detail in the following.…”